A pressure relief valve of this type has become known from DE 38 01 444 A1. The prior-art valve is used in the breathing gas line of an anesthesia apparatus or respirator to make possible the manual as well as the spontaneous respiration by the corresponding switching of a changeover switch. Thus, the valve is opened in the “manual respiration” mode by a possible overpressure in the breathing gas line against a preset closing force in order to release excess gas. The closing force is set by means of a handwheel and a valve spindle, and, depending on the position of the handwheel, a valve spring is compressed more or less strongly in order to thereby vary the opening pressure. In the “spontaneous respiration” mode, the closing element is released, by contrast, by the changeover switch, so that the closing force does not act any longer and the breathing gas can flow off without an appreciable expiration resistance. If the changeover switch is again shifted in the “manual respiration” direction, the original closing force again becomes established without corrections having to be made on the handwheel of the valve.
If complete pressure relief of the breathing gas line must be briefly performed during the manual respiration, this can be performed only if the changeover switch is shifted in the “spontaneous respiration” direction. However, it may now happen, especially when other settings also have to be performed on the anesthesia apparatus or respirator, that the operator forgets to shift the changeover switch to the “manual respiration” position. Delays may thus arise for the user in terms of the continuation of the manual respiration.